Modern communication systems increasingly employ modulation methods such as 8-PSK, 16-QAM, 64-QAM, and OFDM to increase data speeds and to improve spectrum efficiency. This added complexity invariably increases the amplitude (or envelope) fluctuations of the transmit signal—generally measured by the transmit signal's peak-to-average (pk/ave) ratio. As a result, the radio transmitter sees higher peaks and tends to generate more distortion. To minimize distortion, most linear circuits operate at a bias current proportional to the largest amplitude or peak of the transmit signal. This can be very inefficient in systems that produce signals with a large peak-to-average ratio.
A radio transmitter generally uses a power amplifier to close the link to the receiver. The power amplifier typically dissipates more power than any other circuit so its efficiency is critical. It would therefore be advantageous to have a system for operating a power amplifier at lower power levels while keeping distortion low.